Download Neutral ionic liquid [BMIm]BF4 promoted highly selective

Journal of Molecular Catalysis A: Chemical 246 (2006) 70–75
Neutral ionic liquid [BMIm]BF4 promoted highly selective
esterification of tertiary alcohols by acetic anhydride
Zhiying Duan, Yanlong Gu, Youquan Deng ∗
Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Received 23 July 2005; received in revised form 17 October 2005; accepted 17 October 2005
Available online 22 November 2005
Abstract
Esterification of tert-butanol by acetic anhydride, a typical acid–base catalytic reaction, was conducted with excellent selectivity and high yield
in a neutral ionic liquid [BMIm]BF4 (1-butyl-3-methylimidazolium tetrafluoroborate). Many other tertiary alcohols could also be successfully
converted to the corresponding esters in [BMIm]BF4 . Separation of product and recovery and reuse of ionic liquid are all convenient in this system.
Here, the need for catalysts is avoided through the use of catalytically active ionic liquids as solvents.
© 2005 Elsevier B.V. All rights reserved.
Keywords: [BMIm]BF4 ; Esterification; Tertiary alcohol; Acetic anhydride
1. Introduction
Esterification of normal alcohols by carboxylic acids using
homogeneous acid catalysts has been well known [1]. Carboxylic acid esters of tertiary alcohol are useful for the production of agrochemicals, coatings, dyes and solvents [2]. Esters
of unsaturated tertiary alcohol are also important intermediate
in organic synthesis [3]. However, the preparation of carboxylic
acid esters of tertiary alcohols by an acid catalyzed process is
very difficult because of the very high reactivity of tert-alcohols.
Take synthesis of tert-butanol esters, for example, under acidic
condition, tert-butanol undergoes dehydration to iso-butylene,
even at room temperature. Earlier methods used for the synthesis
of tert-butanol esters are either highly cumbersome or involves
the use of highly toxic reagents, such as ketene and acetyl chloride [4–7]. Although tert-butanol could be esterified by acetic
anhydride itself, the major drawback of this method for producing tert-butyl acetate is that the reaction is somewhat sluggish,
since the absence of a catalyst restricts the production rates.
When a catalyst is used to improve the reaction rate, a variety of
unwanted by-products are often produced making the production
of pure tert-butyl acetate more difficult [8]. Recently, Choudhary
and co-workers have reported the highly selective esterification
∗
Corresponding author. Fax: +86 931 4968116.
E-mail address: [email protected] (Y. Deng).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.10.017
of tert-butanol by acetic anhydride using mesoporous Si-MCM41, clay and alumina supported metal Lewis acid, such as InCl3
and GaCl3 , as catalyst [9–11]. However, these metal species are
toxic or expensive. Hence, there is a need to develop an environmentally benign method for the esterification of tert-alcohols.
Ionic liquids are a new class of solvents entirely composed
of ions. Their use as an environmentally friendly alternative
for conventional solvents has gained much attention recently
[12–14]. Esterification reactions have been extensively investigated in many ionic liquids so far, because of the importance
of this transformation both in organic synthesis and chemical industry as well as immiscibility of ester with ionic liquids, which allows easy separation of products. We firstly
reported esterification of carboxylic acids with alcohols in acidic
chloroaluminate ionic liquids [15]. Then, many acidic ionic liquids, including SO3 H-functionalized imidazolium ionic liquids
[16,17], ionic liquids with acidic counteranion and protonated
N-alkylimidazolium ionic liquids have been used in esterification reactions [18,19]. Some particular esterification reactions,
such as transesterification of ␤-ketoesters [20] and synthesis of
ferrocenyl esters [21], could also been conducted in ionic liquids
combined with an acidic catalyst. It is worthy noted that the esterification reactions carried out in ionic liquids, with an exception
of enzyme catalysis, are conducted at acidic conditions in the
most cases. In continuation of our research in esterification, we
report herein, for the first time, highly selective synthesis of tertalcohol acetates from tert-alcohols and acetic anhydride in ionic
Z. Duan et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 70–75
Scheme 1.
liquid [BMIm]BF4 . Different from known catalytic systems, this
reaction could be successfully proceeded in neutral ionic liquid
without any additional acidic catalyst. Functionalized acidic or
basic ionic liquids, however, seems to be unsuitable for this reaction (Scheme 1).
2. Experimental
2.1. Materials and reagents
Anhydrous organic solvents were dried and then distilled
prior to use. tert-Butanol, tert-amyl alcohol, 2-methyl-3-butyn2-ol, 3-methyl-1-pentyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol, 2methyl-3-buten-2-ol and acetic anhydride were not purified prior
to use. All other chemicals used for synthetic procedures were
reagent grade or better.
2.2. Preparation of the ionic liquids
The ionic liquids were synthesized using the methods
described in previously published papers [12,16] and stored in
an exsiccator filled with KOH. Tertiary amine-functionalized
ionic liquid [MEnIm]PF6 is prepared by the reaction of 1methylimidazole with 2-diethylamine ethyl chloride hydrochloride in ethanol. After 24 h under reflux, the ethanol is removed
in vacuum and the solid residue dissolved in a large amount of
water that is brought to pH 8 by the addition, in small portions,
of solid K2 CO3 . Subsequent ion exchange with KPF6 in water
at 60 ◦ C gives the product salt. After drying under vacuum at
80 ◦ C, the product was obtained in 84% yield as a relatively viscous liquid. δH (400 MHz, CD3 COCD3 ) 0.89 (6H, t, J = 6.8 Hz),
2.55 (4H, q, J = 6.8 Hz), 2.81 (2H, t, J1 = 5.6 Hz, J2 = 6.4 Hz),
3.85 (3H, s), 4.22 (2H, t, J1 = 6.4 Hz, J2 = 5.6 Hz), 7.70 (1H, d,
J = 6.4 Hz), 7.72 (1H, d, J = 6.0 Hz), 9.02 (1H, s).
2.2.1. Synthesis of tert-alcohol esters
All reactions were conducted in a 90 ml autoclave with a
glass tube inside equipped with magnetic stirring. In each reaction, 4.5 ml ionic liquid, 10 mmol alcohol and 12 mmol acetic
anhydride were successively introduced and reacted at 60 ◦ C for
desired period. After the reaction, cooled the autoclave to room
temperature (when using tert-butanol as substrate, the autoclave
should be cooled to −40 ◦ C), then the reaction mixture was analyzed by GC. The isolation of organic compounds with ionic
liquid was conducted by direct distillation or vacuum distillation. Purification of desired was performed as follow: (1) remove
acetic acid and residual acetic anhydride by extraction of water
(20 ml × 3) and (2) the hydrophobic organic residue was further purified using a short column of silica gel (2 cm in length,
0.5 cm in diameter, acetate–hexane, 1:9) to afford the pure product. Qualitative and quantitative analyses of organic products
71
were conducted with a HP 6890/5973 GC/MS and a HP 1790
GC equipped with a FID detector. The concentration of reactant
and product was directly given by the system of GC chemstation
according to the area of each chromatograph peak.
Tert-butyl acetate: Colorless liquid (1.04 g, 88%); δH
(300 MHz, CD3 COCD3 ) 1.42 (9H, s), 1.89 (3H, s); ν (cm−1 )
1739 (C O), 1265 and 1173 (O C O); GC–MS: 101, 78,57,
43. Anal. Calcd for C6 H12 O2 (116.16): C, 62.04; H, 10.41.
Found: C, 62.15; H, 10.67.
Tert-amyl acetate: Colorless liquid (1.12 g, 87%); δH
(300 MHz, CD3 COCD3 ) 0.87 (3H, t, J = 7.7 Hz), 1.39 (3H, s),
1.73–1.80 (2H, m), 1.96 (3H, s); ν (cm−1 ) 1736 (C O), 1261
and 1161 (O C O); GC–MS: 115, 101, 55, 43. Anal. Calcd
for C7 H14 O2 (128.17): C, 65.59; H, 11.01. Found: C, 65.74;
H, 10.87.
2-Methyl-3-butyn-2-acetate: Colorless liquid (1.11g, 88%); δH
(300 MHz, CD3 COCD3 ) 1.63 (6H, s), 1.95 (3H, s), 2.94(1H,
s); ν (cm−1 ) 1742 (C O), 3291 (C H), 2119 (C C), 1248 and
1137 (O C O); GC–MS: 111, 98, 83, 67, 43; Anal. Calcd for
C7 H10 O2 (126.15): C, 66.64; H, 7.99. Found: C, 66.91; H, 7.87.
3-Methyl-1-pentyn-3-acetate: Colorless liquid (1.11g, 83%);
δH (300 MHz, CD3 COCD3 ) 0.99 (3H, t, J = 5.4 Hz), 1.60 (3H,
s), 1.79–2.01(2H, m), 1.95 (3H, s), 2.96 (1H, s); ν (cm−1 ) 1745
(C O), 3289 (C H), 2114 (C C), 1243 and 1128 (O C O);
GC–MS: 125, 111, 97, 79, 69, 53; Anal. Calcd for C8 H12 O2
(140.18): C, 68.54; H, 8.63. Found: C, 68.68; H, 8.78.
3,5-dimethyl-1-hexyn-3-acetate: Colorless liquid (1.36g,
81%); δH (300 MHz, CD3 COCD3 ) 0.97–1.00 (6H, m), 1.66
(3H, s), 1.71–1.88(2H, m), 1.96–2.01(1H, m), 1.95 (3H, s),
2.89 (1H, s); ν (cm−1 ) 1747 (C O), 3288 (C H), 2117 (C C),
1240 and 1138 (O C O); GC–MS: 153, 125, 111, 93, 84, 77,
69; Anal. Calcd for C10 H16 O2 (168.23): C, 71.39; H, 9.59.
Found: C, 71.49; H, 9.68.
2.3. Characterization procedures
The FTIR spectra of the ionic liquids were recorded on NicoLET 5700 Fourier transform infrared spectrophotometer using
pyridine as a probe for acidity measurement. The pyridine used
was dried with KOH and distilled over 5 Å molecular sieves
and solid KOH. The IR samples were prepared by mixing pyridine with ionic liquid in a given mass ratio of pyridine/ionic
liquid = 1:3, and then spreading into liquid films on KBr windows. 1 H NMR (300 MHz) spectra of products were obtained
as solutions in CD3 COCD3 . Chemical shifts were reported in
parts per million (ppm, δ) and referenced to TMS.
3. Results and discussion
3.1. Effect of different ionic liquids on the reactivity
Because the esterification of tert-butanol by acetic anhydride
has considered as a typical acid-catalytic reaction, acidic or
basic ionic liquids may be effective catalysts for the synthesis
tert-butyl acetate. In our previous works, SO3 H-functionalized
imidazolium ionic liquids have been used as an effective catalyst
Z. Duan et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 70–75
72
Scheme 2. Functionalized ionic liquids used in the esterification of tert-butanol
by acetic anhydride.
for the addition of carboxylic acids to olefins and the oligomerization of branched olefins [22,23]. Thus, the esterification of
tert-butanol was primarily investigated in this kind of ionic liquid
([MBsIm][TfO], Scheme 2). However, a lot of iso-butene and its
oligomers (branched olefins of C8 and C12) were detected in the
reaction mixture and the desired tert-butyl acetate was obtained
in a selectivity of 13% although tert-butanol was completely
consumed after 4 h reaction (Table 1, Entry 1). It indicated that
dehydration of tert-butanol to produce iso-butene and the consequent oligomerization of iso-butene are predominant reactions in
the present conditions. Subsequently, another protonated acidic
ionic liquid [HMIm]BF4 (in Scheme 2), which has proved to be
a green and cost-effective acidic catalyst in many organic reactions [24,25], was evaluated in the esterification of tert-butanol
under the same conditions. Same as observed in ionic liquid
[MBsIm][TfO], the conversion of tert-butanol was completely
consumed, however, selectivity to tert-butyl acetate was only
37% (Entry 2). Although the oligomerization of iso-butene was
inhibited in certain case, tert-butanol was still readily dehydrated
to iso-butene in [HMIm]BF4 .
Synthesis tert-butyl acetate from tert-butanol and acetic
anhydride could also be promoted by a basic catalyst [8]. If
the imidazolium cation of ionic liquid was modified with a tertiary amine, it should be a weaker basic medium ([MEnIm]PF6 ,
Scheme 2). In the above esterification, this new ionic liquid represents a better activity compared with the former two acidic
ionic liquids. Conversion of tert-butanol and selectivity to tertbutyl acetate are all more than 70% (Table 1, Entry 3). However,
the reaction mixture became a white and highly viscous colloid
after the reaction, and the ionic liquid probably reacted with the
produced acetic acid.
Our previous investigations on the conversion of tert-butanol
revealed that the reactivity of tert-butanol could be greatly
improved by the use of tetrafluoroborate ionic liquids as media
[26,27]. Therefore, we consequently investigated the esterification reaction of tert-butanol in [BMIm]BF4 , which is considered
as a neutral polar solvent. To our surprising, 93% conversion and
99% selectivity to tert-butyl acetate were obtained under the
identical conditions (Table 1, Entry 4). This unusual observation promoted us to further investigate the effects of other ionic
liquids on the esterification reaction. The rate of the formation of
tert-butyl acetate in [BMIm]BF4 at 60 ◦ C seemed to be slightly
higher than that observed with [EMIm]BF4 , [BMIm]NO3 and
[BMIm][PhSO3 ]. It is important to note that the selectivities to
tert-butyl acetate are all close to 100% in these ionic liquids
(Entries 5–7). When using [BMIm]PF6 as medium, a lot of isobutene was detected beside desired tert-butyl acetate (Entry 8). It
may contribute to the acidic hydrofluoric acid (HF) which comes
from the decomposition of [BMIm]PF6 [28], thus tert-butanol
could be readily dehydrated to iso-butene.
To determine whether the use of ionic liquid [BMIm]BF4
as medium was an essential factor to realize the high conversion and excellent selectivity, the same reaction was carried
out in absence of medium or in traditional organic solvents.
As described in Table 1, the conversion of tert-butanol was only
14% without solvent, and 9% in the present of organic solvent
at the best (Table 1, Entries 9–12). These results clearly show
the determining role of [BMIm]BF4 in this reaction.
3.2. Optimization of reaction conditions
Consequently, the reaction condition was optimized using
[BMIm]BF4 as solvent, and the results were listed in Table 2.
Theoretic ratio of tert-butanol to acetic anhydride is 1:1 in this
reaction. However, the magnitude of tert-butanol conversion was
only 78% when using equal amount of tert-butanol and acetic
Table 1
Scanning of various ionic liquids on esterification of tert-butanol by acetic anhydridea
Entry
Solvents
Conversion of alcohol (%)
Selectivity to ester (%)
1
2
3
4
5
6
7
8
9
10
11
12
[MBsIm][TfO]
[HMIm]BF4
[MEnIm]PF6
[BMIm]BF4
[EMIm]BF4
[BMIm]NO3
[BMIm][PhSO3 ]
[BMIm]PF6
–
CH3 CN
Toluene
CHCl3
100
100
72
93
87
80
82
99
14
9
5
6
13
37
71
99
99
99
99
61
100
99
100
100
a
Solvent, 4.5 ml; tert-butanol, 0.74 g; acetic anhydride, 1.22 g, reaction temperature, 60 ◦ C; reaction time, 4 h.
Z. Duan et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 70–75
73
Table 2
Condition optimization of the esterification in ionic liquid [BMIm]BF4 a
Entry
Ratio of anhydride to alcohol (M/M)
Temperature (◦ C)
Reaction time (h)
Conversion of alcohol (%)
Selectivity to ester (%)
1
2
3
4
5
6
7
1:1
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
60
60
45
75
60
60
60
4
4
4
4
2
3
5
78
93
59
99
58
86
95
99
99
99
87
99
99
99
a
[BMIm]BF4 , 4.5 ml; tert-butanol, 0.74 g.
anhydride after 4 h reaction at 60 ◦ C (Entry 1). A satisfactory
result could be obtained using an anhydride/alcohol ratio of 1.2.
Conversion of tert-butanol would be decreased with the decrease
of reaction temperature or the reaction time (Entries 3, 5 and 6).
Although the reactivity of tert-butanol could be improved when
the reaction temperature was elevated to 75 ◦ C, the formation
of iso-butene could also be indulged; therefore, the selectivity
was disturbed (Entry 4). It is known that the dehydration of tertbutanol to iso-butene is a reversible process, therefore, it is not
surprising that higher temperature is the booster to render this
type of reaction toward formation of iso-butene. When the reaction time was further increased, the conversion of tert-butanol
almost keeps constant and as a result, 4 h seems to be suitable
for this reaction in [BMIm]BF4 .
3.3. Esterification of C4 alcohols with acetic anhydride in
[BMIm]BF4
The reactivity of primary and secondary alcohols has also
been investigated in our system, and the results are listed in
Table 3. n-Butanol and 2-methyl-1-propanol could be esterified by acetic anhydride in [BMIm]BF4 , however, the reactions
seems to be sluggish and the conversions of alcohols were less
than 35% even the reaction time was prolonged to 10 h. In the
case of 2-butanol, the corresponding ester could be obtained
in 55% of alcohol conversion. Different from reaction of tertbutanol, ester was the sole product in the reactions of primary and
secondary alcohols. Ranu et al. reported the yields of acylation
reactions of alcohols by acetic anhydride could be improved by
removal of produced acetic acid [29]. In our system, the conversion of primary and secondary alcohols may also be increased
to some extent by analogous technology.
3.4. Esterification of acid–unstable alcohols in ionic liquid
[BMIm]BF4
Having these results in hand, other tertiary alcohols have been
subjected to the conditions of Table 3. It is worthy noted that
the produced ester and other organic compounds (the produced
acetic acid, un-reacted acetic anhydride and alcohol) in our systems could be simply separated form the reaction mixture by
distillation under reduced pressure. This allows easy recovery
and reuse of ionic liquid [BMIm]BF4 . From Table 4, tert-butyl
acetate was obtained in an isolated yield of 88% (Entry 1) by distillation and consequent column chromatography. Then, another
1 equiv. of alcohol and 1.2 equiv. of acetic anhydride were added
to the recovered ionic liquid. Similar results were obtained even
ionic liquid [BMIm]BF4 was reused in the fifth time (Table 4,
Entry 7). Under the same conditions, tert-amyl alcohol could
be esterified by acetic anhydride to produce the corresponding acetate with a yield of 87% (Entry 2). Tertiary propargylic
alcohols have been considered are acid-unstable and tend to
polymerize in [BMIm]PF6 , which reveals acidic under moisture
conditions from our previous investigation [30]. Fortunately,
these alcohols could readily be esterified by acetic anhydride
in ionic liquid [BMIm]BF4 , and the yields of them are all more
than 80% (Entries 3–5). It should be pointed that the produced
propargylic acetates are important synthetic intermediates [3],
Table 3
Esterification of C4 alcohols by acetic anhydride in ionic liquid [BMIm]BF4 a
Entry
Reaction time (h)
Conversion of alcohol (%)
Selectivity to ester (%)
1
10
30
100
2
10
34
100
3
4
55
100
4
4
93
99
a
Alcohols
[BMIm]BF4, 4.5 ml; alcohol, 0.74 g; acetic anhydride, 1.22 g, reaction temperature, 60 ◦ C.
Z. Duan et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 70–75
74
Table 4
Esterification of acid–unstable alcohols in ionic liquid [BMIm]BF4
a
Entry
Alcohols
Products
Reaction time (h)
Yields (%)
1
4
88
2
4
87
3
4
88
4
6
83
5
10
81
6
4
68b
4
87
7c
a
b
c
[BMIm]BF4 , 4.5 ml; alcohol, 10 mmol; acetic anhydride, 1.22 g, reaction temperature,
GC yield.
Reused in the fifth time.
therefore, this esterification process in [BMIm]BF4 should be
a valuable method in organic synthesis. When using 2-methyl3-buten-2-ol, which could be easily dehydrated to produce 2methyl-1,3-butadiene under acidic condition, as substrate, the
yield of ester was 68% (Entry 6). These results indicated that
our method is applicable in esterification of many tertiary alcohols.
3.5. [BMIm]BF4 is a neutral ionic liquid before and after
the esterification reaction
Water is present in ionic liquids of the type we studied, even
after a moderate drying procedure [31]. Neutral [BMIm]PF6 liberates traces of HF when it comes in contact with moisture or
heated at high temperature [28]. Considering the esterification
being a typical acid-catalysis reaction and the high reactivity
of [BMIm]BF4 ionic liquid, we preliminarily investigated the
acidic property of the used [BMIm]BF4 . After the reaction, the
PH test paper displayed pale green color in the presence of the
reaction mixture indicative of neutral of [BMIm]BF4 . Alternatively, the Lewis and Brønsted acidity of the ionic liquid has been
determined using pyridine as a probe molecule by monitoring
the bands in the range of 1400–1700 cm−1 arising from its ring
vibration modes [32,33]. The FTIR spectra of neat pyridine show
a band at 1437 cm−1 (Fig. 1). On mixing pyridine with ionic liquid [BMIm]BF4 (both the fresh and the fifth used ionic liquids
are tested), the spectra of the two cases show no band near 1450
or 1540 cm−1 , which indicate the presence of the Lewis acid
sites and Brønsted acid sites respectively. Then, we believe that
the [BMIm]BF4 is a neutral ionic liquid before and after the reac-
60 ◦ C.
tion. As to the reaction mechanism of this process, it is not clear
at this stage. The understanding of the mechanism of catalysis
in ionic liquids is still in its infancy. The ionic liquids are composed only of ions, such ionic nature can affect the course of the
reactions. It can be conjectured that the strong electrostatic field
possessed by the dialkyl imidazolium cation mediated with the
counterion, may play an important role in initiating the desired
reaction and a traditional acid or base catalytic mechanism was
not involved. It should be pointed out here, using the viewpoint
that [BMIm]BF4 would be decomposed as a acidic medium to
explain the high reactivity of tert-butanol, especially the excellent selectivity to the correspongding iso-butene acetate seems
to be unreasonable.
Fig. 1. FT-IR spectra of (a) pure pyridine; (b) pyridine + fresh [BMIm]BF4 ; (c)
pyridine + the fifth used [BMIm]BF4 (pyridine/ionic liquid = 1:3 by weight in (b
and c).
Z. Duan et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 70–75
4. Conclusion
In conclusion, we have developed an unusual esterification
method for acid unstable tertiary alcohols using neutral ionic
liquid [BMIm]BF4 as highly selective catalyst and solvent. It
should be particularly pointed out that some functionalized
acidic or basic ionic liquids are all less selective in the same conditions. Many functionalized tertiary alcohols, such as propargylic alcohols and allyl alcohol could be successfully converted
to the corresponding acetates with good to excellent isolated
yields in our system. This simple and effective process is attractive for the chemical industry and green organic synthesis.
Acknowledgement
This work was financially supported by National Natural Science Foundation of China (No. 20225309, 20533080).
References
[1] S. Patai (Ed.), The Chemistry of Carboxylic Acid and Esters, New York,
Wiley, 1969.
[2] (a) G.W. Anderson, F.M. Callaham, J. Am. Chem. Soc. 82 (1960) 3359;
(b) K.A. Williams, S.L. Cook, P.W. Turner, US Patent 6,657,075 (2003).;
(c) R. Kroker, G. Nestler, W. Schmitt, W. Schumm, US Patent 6,756,506
(2004).
[3] (a) H. Okumoto, S. Nishihara, H. Nakagawa, A. Suzuki, Synlett 217
(2000);
(b) K. Kato, Y. Yamamoto, H. Akita, Tetrahedron Lett. 43 (2002) 6587.
[4] E.M. Kaiser, R.A. Woodruff, J. Org. Chem. 35 (1970) 1198.
[5] S. Takimoto, J. Inanaga, J. Katsuki, M. Yamaguchi, Bull. Chem. Soc.
Jpn. 49 (1976) 2335.
[6] A. Armstrong, I. Brackenridge, R.F.W. Jackson, J.M. Kirk, Tetrahedron
Lett. 29 (1988) 2483.
[7] K. Nagasawa, S. Yoshitake, T. Amiya, K. Ito, Synth. Commun. 20 (1990)
2033.
75
[8] K. Nagasawa, K. Ohhashi, A. Yamashita, K. Ito, Chem. Lett. (1994)
209.
[9] V.R. Choudhary, K. Mantri, S.K. Jana, Microporous Mesoporous Mater.
47 (2001) 179.
[10] V.R. Choudhary, K. Mantri, S.K. Jana, Catal. Commun. 2 (2001) 57.
[11] M. Salavati-Niasari, T. Khosousi, S. Hydarzadeh, J. Mol. Catal. A:
Chem. 235 (2005) 150.
[12] R. Sheldon, Chem. Commun. (2001) 2399.
[13] H. Olivier-Bourbigou, L. Magan, J. Catal. Mol. A: Chem. 182–183
(2002) 419.
[14] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2002) 3772.
[15] Y. Deng, F. Shi, J. Peng, K. Qiao, J. Mol. Catal. A Chem. 165 (2001)
33.
[16] (a) A.C. Cole, J.L. Jensen, I. Ntai, K.L.T. Tran, K.J. Weave, D.C. Forbes,
J.H. Davis, J. Am. Chem. Soc. 124 (2002) 5962;
(b) D.C. Forbes, K.J. Weaver, J. Mol. Catal. A Chem. 214 (2004) 129;
(c) J.H. Davis Jr., Chem. Lett. 33 (2004) 1072.
[17] K. Qiao, C. Yokoyama, Chem. Lett. 33 (2004) 472.
[18] J. Fraga-Dubreuil, K. Bourahla, M. Rahmouni, J.P. Bazureau, J.
Hamelin, Catal. Commun. 3 (2002) 185.
[19] H.P. Zhu, F. Yang, J. Tang, M.Y. He, Green Chem. 5 (2003) 38.
[20] B. Wang, L.M. Yang, J.S. Suo, Tetrahedron Lett. 44 (2003) 5037.
[21] C. Imrie, E.R.T. Elago, C.W. McCleland, N. Williams, Green Chem. 4
(2002) 159.
[22] Y. Gu, F. Shi, Y. Deng, J. Mol. Catal. A Chem. 212 (2004) 71.
[23] Y. Gu, F. Shi, Y. Deng, Catal. Commun. 4 (2003) 597.
[24] G. Zhao, T. Jiang, H. Gao, B. Han, J. Huang, D. Sun, Green Chem. 6
(2004) 75.
[25] H.H. Wu, F. Yang, P. Cui, J. Tang, M.Y. He, Tetrahedron Lett. 45 (2004)
4963.
[26] F. Shi, H. Xiong, Y. Gu, S. Guo, Y. Deng, Chem. Commun. (2003)
1054.
[27] K. Qiao, Y. Deng, N. J. Chem. (2002) 667.
[28] H. Shen, Z.M.A. Judeh, C.B. Ching, Tetrahedron Lett. 44 (2003) 981.
[29] B.C. Ranu, S.S. Dey, A. Hajra, Green Chem. 5 (2003) 44.
[30] Y. Gu, F. Shi, Y. Deng, J. Org. Chem. 69 (2004) 391.
[31] M. Kosmulski, J. Gustafsson, J.B. Rosenholm, Thermochim. Acta 412
(2004) 47.
[32] E.P. Parry, J. Catal. 2 (1963) 371.
[33] Y. Yang, Y. Kou, Chem. Commun. 226 (2004) 227.